1. Field of the Invention
The present invention relates to a method and arrangement for determining the location of an active (i.e. a measurement and/or treatment) catheter within an animal (including human) body and in particular to such a method and arrangement of the type employing triangulation techniques in order to make the determination.
2. Description of the Prior Art
Systems for locating a catheter within an animal body using triangulation are well known and such a system is described, for example, in U.S. Pat. No. 5,042,486. In this known system, an electromagnetic or acoustic signal is transmitted between an antenna at the tip of an active catheter and three reference antennas placed on the outside of the animal. The distances between each reference antenna and the catheter antenna are then combined, using triangulation methods, to provide a determination of the active catheter in three-dimensions.
For many medical applications it is desirable to be able to locate an active catheter in a patient, e.g. in angiographic examinations and in cardiac diagnostics and therapy, with as high a degree of accuracy as possible.
One system that is used to locate such an active catheter with an improved accuracy is disclosed in PCT Publication WO 98/00060, and has a fixed reference catheter and an active catheter, between which acoustic or electromagnetic signals are transmitted. The transmitted signals are used to measure the distance between transducers mounted on the reference catheter and at least one transducer mounted on the active catheter. The location of the catheter is determined by triangulation. In the system described in WO 98/00060 both the reference and the active catheter are positioned inside the patient, with the consequence that the position measurements may be made more accurately since they are not affected by the movement or breathing of the patient. WO 98/00060 also describes a system in which the number of transducers that are mounted on the reference catheter exceeds the minimum number of transducers required mathematically to make the location determination. In this way a number of independent determinations of the catheter location may be made by using a different combination of reference transducers for each determination. An averaged location may then be calculated having an increased accuracy over a single determination.
Similarly, WO 96/31753 describes a digital ultrasound catheter tracking system that can employ up to 32 transducers to create a redundancy in distance measurements so that the three-dimensional location of the catheter can be determined even if some distance measurements are ignored, for example because of poor signal propagation through the measurement volume.
Nevertheless, even when using a system in which a greater number of distances are measured than are mathematically required to locate the catheter, the determined location may still have a relatively large error associated with it.